Semiconductor storage device

ABSTRACT

A semiconductor storage device includes an SRAM memory cell composed of a drive transistor, a transfer transistor and a load transistor, an I/O circuit that is connected to bit lines connected to the memory cell, and an operating mode control circuit that switches an operating mode of the I/O circuit between a resume standby mode and a normal operation mode, wherein the I/O circuit includes a write driver that writes data to bit lines, a sense amplifier that reads data from the bit lines, a first switch inserted between the bit lines and the write driver, a second switch inserted between the bit lines and the sense amplifier, a precharge circuit that precharges the bit lines, and a control circuit that controls the first and second switches and the precharge circuit according to a signal from the operating mode control circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2014-040521, filed on Mar. 3, 2014, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to a semiconductor storage device.

In SRAM (Static Random Access Memory), which is one of semiconductorstorage devices, various proposals are made for reduction of a leakagecurrent.

For example, a technique that reduces a leakage current by raising thesource potential of a memory cell to be higher than the VSS level duringSRAM resume standby is proposed (Japanese Unexamined Patent ApplicationPublication No. 2004-206745). In this technique, 0.4V is applied to thesource of the memory cell. On the other hand, 1.0V is applied as a powersupply potential to bit lines.

Besides, a technique that sets bit lines in floating state during resumestandby in order to prevent an excessive leakage current from flowingdue to a hardware defect such as fixation of a memory cell internal nodeto Low level is proposed (Japanese Unexamined Patent ApplicationPublication No. 2010-198729).

In the resume standby mode of a resume standby circuit, a channelleakage is reduced by raising the source potential of a memory cell tobe higher than the VSS level, thereby reducing a leakage current of thewhole module. In this mode, a voltage at the VDD level or the levellower than VDD by NMOS Vth is applied to the bit lines. On the otherhand, in the recent microfabrication process, a leakage current to thesubstrate of an access transistor through the bit lines is large due toGIDL (Gate Induced Drain Leakage), and particularly at room temperature,a leakage current cannot be sufficiently reduced in a normal resumestandby circuit.

SUMMARY

The present inventor has found that the above-described techniques havethe following problems. In the recent microfabrication process, aleakage current to the substrate of an access transistor through bitlines is not negligible due to GIDL (Gate Induced Drain Leakage).Particularly, at room temperature, the GIDL component is dominantcompared with the channel leakage component. Therefore, in the resumestandby circuit that raises the source potential of the memory cell tobe higher than the VSS level disclosed in Japanese Unexamined PatentApplication Publication No. 2004-206745, the leakage current cannot beeffectively reduced at room temperature. Further, if the bit lines areset to floating during resume standby as disclosed in JapaneseUnexamined Patent Application Publication No. 2010-198729, a leakagecurrent through the bit lines due to GIDL, not only a hardware defect,can be reduced. However, the source potential of the memory cell is atthe VSS level in Japanese Unexamined Patent Application Publication No.2010-198729, and a leakage current cannot be effectively reduced at hightemperature. Another problem of setting the bit lines to floating is anincrease in a peak current at time of resume return. If the bit linesare set to floating, the bit line potential decreases to the VSS leveldue to a leakage current or the like in some cases. When returning fromthe resume standby mode to the normal operation mode, the bit lines arecharged from the VSS level to the VDD level by a precharge transistor.In the normal operation, the number of bit lines to be charged is onebit line pair for each MUX (Y-address multiplexer) and either one ofTrue/Bar, and thus the number of bit lines to be charged at a time islimited to the number of all bit lines/MUX/2. On the other hand, whenreturning from the resume standby mode to the normal operation mode,there is a possibility that all bit lines are charged at the same time.Because the precharge transistor needs to charge the bit lines to theVDD level in one cycle during the normal operation, it is designed tohave a considerably large size. Therefore, if the precharge transistorcharges all bit lines at the same time, a significantly large peakcurrent flows, which can cause the occurrence of an instantaneousvoltage drop. FIG. 11 is a diagram schematically showing a voltage dropduring precharge of a semiconductor storage device. A voltage drop cancause the occurrence of a malfunction in another analog circuit, logiccircuit or the like in the vicinity, for example. Further, it can causea reliability defect such as electromigration.

The other problems and novel features of the present invention willbecome apparent from the description of the specification and theaccompanying drawings.

A semiconductor device according to one embodiment includes an SRAMmemory cell, an I/O circuit connected to bit lines, and an operatingmode control circuit that switches an operating mode of the I/O circuit.The I/O circuit includes a write driver, a sense amplifier, a firstswitch inserted between the bit lines and the write driver, a secondswitch inserted between the bit lines and the sense amplifier, aprecharge circuit that precharges the bit lines, and a control circuitthat controls the first and second switches and the precharge circuit.The control circuit turns off the first and second switches and theprecharge circuit in the resume standby mode, and causes the prechargecircuit to precharge the bit lines with a smaller driving force comparedwith in the normal operation mode when returning from the resume standbymode to the normal operation mode.

According to one embodiment, it is possible to reduce a leakage currentand suppress a bit line precharge current when switching operating modein a semiconductor storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram schematically showing a configuration of asemiconductor storage device according to a first embodiment.

FIG. 2 is a circuit diagram showing in more detail a configuration ofthe semiconductor storage device according to the first embodiment.

FIG. 3 is a diagram showing a configuration example of a delay circuit.

FIG. 4 is a timing chart of signals in the semiconductor storage deviceaccording to the first embodiment.

FIG. 5 is a circuit diagram schematically showing a configuration of asemiconductor storage device according to a second embodiment.

FIG. 6 is a block diagram schematically showing a configuration of asemiconductor storage device according to a third embodiment.

FIG. 7 is a circuit diagram showing a word line driver and a memory cellaccording to the third embodiment.

FIG. 8 is a timing chart of signals in the semiconductor storage deviceaccording to the third embodiment.

FIG. 9 is a circuit diagram schematically showing a configuration of asemiconductor storage device according to a fourth embodiment.

FIG. 10 is a timing chart of signals in the semiconductor storage deviceaccording to the fourth embodiment.

FIG. 11 is a diagram schematically showing a voltage drop duringprecharge in a semiconductor storage device.

DETAILED DESCRIPTION

The preferred embodiments of the present invention will be describedhereinafter in detail with reference to the drawings. It is noted thatin the description of the drawings the same elements will be denoted bythe same reference symbols and redundant description will be omitted.

First Embodiment

A semiconductor storage device 100 according to a first embodiment isdescribed hereinafter. FIG. 1 is a block diagram schematically showing aconfiguration of the semiconductor storage device 100 according to thefirst embodiment. FIG. 2 is a circuit diagram showing in more detail aconfiguration of the semiconductor storage device 100 according to thefirst embodiment. As shown in FIGS. 1 and 2, the semiconductor storagedevice 100 is configured as SRAM. The semiconductor storage device 100includes a memory cell 1, an I/O circuit 2, and an operating modecontrol circuit 3.

The semiconductor storage device 100 includes a plurality of memorycells, a plurality of word lines, and a plurality of bit line pairs.Note that, however, because the configuration of the memory cells, theword lines and the bit line pairs are respectively the same, each of theplurality of memory cells, the plurality of word lines, and theplurality of bit line pairs is not distinguished from one another in thefollowing embodiments.

The memory cell 1 includes NMOS transistors N1 to N4 and PMOStransistors P1 and P2. In the memory cell 1, the NMOS transistors N1 andN2 function as transfer transistors. The NMOS transistors N3 and N4function as drive transistors. The PMOS transistors P1 and P2 functionas loads.

The drain of the NMOS transistor N1 is connected to the bit line BT. Thedrain of the NMOS transistor N2 is connected to the bit line BB. Thegates of the NMOS transistors N1 and N2 are connected to the word lineWL. A power supply potential VDD is applied to the sources of the PMOStransistors P1 and P2. The drain of the PMOS transistor P1 is connectedto the source of the NMOS transistor N1, the drain of the NMOStransistor N3 and the gates of the NMOS transistor N4 and the PMOStransistor P2. The drain of the PMOS transistor P2 is connected to thesource of the NMOS transistor N2, the drain of the NMOS transistor N4,and the gates of the NMOS transistor N3 and the PMOS transistor P1. Thesources of the NMOS transistors N3 and N4 are connected to the ground(ground potential VSS).

The I/O circuit 2 includes a write driver 21, a sense amplifier 22, anormal operation precharge circuit 23, a resume standby return prechargecircuit 24, a write column switch 25, a read column switch 26, and acolumn I/O control circuit 27.

The write driver 21 writes data to the bit line BT and the bit line BB.The sense amplifier 22 reads data from the bit line BT and the bit lineBB.

The normal operation precharge circuit 23 includes PMOS transistors P31to P33. One end of the PMOS transistor P31 is connected to the bit lineBT and the other end is connected to the bit line BB. The power supplypotential VDD is applied to the sources of the PMOS transistors P32 andP33. The drain of the PMOS transistor P32 is connected to the bit lineBT. The drain of the PMOS transistor P33 is connected to the bit lineBB. A precharge control signal CPC is input to the gates of the PMOStransistors P31 to P33 from the column I/O control circuit 27.

The resume standby return precharge circuit 24 includes PMOS transistorsP41 and P42. The power supply potential VDD is applied to the sources ofthe PMOS transistors P41 and P42. The drain of the PMOS transistor P41is connected to the bit line BT. The drain of the PMOS transistor P42 isconnected to the bit line BB. A resume mode return precharge signal RSPCis input to the gates of the PMOS transistors P41 and P42 from theoperating mode control circuit 3.

The write column switch 25 includes NMOS transistors N51 and N52. Oneend of the NMOS transistors N51 is connected to the bit line BT and theother end is connected to the write driver 21. One end of the NMOStransistors N52 is connected to the bit line BB and the other end isconnected to the write driver 21. A write switch control signal CWSE isinput to the gates of the NMOS transistors N51 and N52 from the columnI/O control circuit 27.

The read column switch 26 includes PMOS transistors P61 and P62. One endof the PMOS transistor P61 is connected to the bit line BT and the otherend is connected to the sense amplifier 22. One end of the PMOStransistor P62 is connected to the bit line BB and the other end isconnected to the sense amplifier 22. A read switch control signal CRSEis input to the gates of the PMOS transistors P61 and P62 from thecolumn I/O control circuit 27.

The column I/O control circuit 27 includes a PMOS transistor P71, NANDcircuits 271 and 272 and an inverter 273. The power supply potential VDDis applied to the source of the PMOS transistor P71. An invertedoperating mode switching signal RSI is input to the gate of the PMOStransistor P71. A precharge signal PC is input to one input terminal ofthe NAND circuit 271, and the output terminal is connected to the gatesof the PMOS transistors P31 to P33 in the normal operation prechargecircuit 23 and outputs the precharge control signal CPC. A Y-selectionsignal Y0 is input to one input terminal of the NAND circuit 272, and asense enable signal SE from the sense amplifier 22 is input to the otherinput terminal. The output terminal of the inverter 273 is connected tothe gates of the NMOS transistors N51 and N52 in the write column switch25 and outputs the write switch control signal CWSE. The drain of thePMOS transistor P71, the gates of the PMOS transistors P61 and P62 inthe read column switch 26, the other input terminal of the NAND circuit271, the output terminal of the NAND circuit 272 and the input terminalof the inverter 273 are connected to each other.

The operating mode control circuit 3 includes an inverter 31, a delaycircuit 32, an AND circuit 33, an inverter 34 and a NAND circuit 35. Anoperating mode switching signal RS is input to the input terminal of theinverter 31, and an inverted operating mode switching signal RSI, whichis an inverted signal of the operating mode switching signal RS, isoutput from the output terminal. The input terminal of the delay circuit32 is connected to the output terminal of the inverter 31 and receivesthe inverted operating mode switching signal RSI. A delayed invertedoperating mode switching signal RSI_D, which is generated by delayingthe inverted operating mode switching signal RSI, is output from theoutput terminal of the delay circuit 32. One input terminal of the ANDcircuit 33 is connected to the output terminal of the inverter 31 andreceives the inverted operating mode switching signal RSI. The otherinput terminal of the AND circuit 33 is connected to the output terminalof the delay circuit 32 and receives the delayed inverted operating modeswitching signal RSI_D. The precharge signal PC is output from theoutput terminal of the AND circuit 33. The input terminal of theinverter 34 is connected to the output terminal of the delay circuit 32and receives the delayed inverted operating mode switching signal RSI_D,and the output terminal is connected to one input terminal of the NANDcircuit 35. The other input terminal of the NAND circuit 35 is connectedto the output terminal of the inverter 31 and receives the invertedoperating mode switching signal RSI. The output terminal of the NANDcircuit 35 is connected to the gates of the PMOS transistors P41 and P42in the resume standby return precharge circuit 24 and outputs the resumemode return precharge signal RSPC. Further, the inverted operating modeswitching signal RSI is output to the gate of the PMOS transistor P71 inthe column I/O control circuit 27.

Note that the delay circuit 32 can be configured as follows, forexample. FIG. 3 is a diagram showing a configuration example of thedelay circuit 32. The delay circuit 32 includes buffers 321, an inverter322 and an inverter 323.

The inverter 322 is placed in the position where it can receive supplyof the inverted operating mode switching signal RSI having passedthrough the memory cell in the semiconductor storage device 100. In thisposition, the inverted operating mode switching signal RSI is input tothe input terminal of the inverter 322.

The buffers 321 are placed respectively in near proximity to theplurality of I/O circuits 2 corresponding to the plurality of memorycells 1 in the semiconductor storage device 100. The plurality ofbuffers 321 are connected in cascade. The input terminal of theplurality of buffers 321 connected in cascade is connected to the outputterminal of the inverter 322. The output terminal of the plurality ofbuffers 321 connected in cascade is connected to the input terminal ofthe inverter 323. The delayed inverted operating mode switching signalRSI_D is output from the output terminal of the inverter 323.

The operation of the semiconductor storage device 100 is describedhereinafter. FIG. 4 is a timing chart of signals in the semiconductorstorage device 100 according to the first embodiment. The operation inthe case of NOP (non-operation) state in the normal operation mode isdescribed first. In this state, the word line WL is Low level, theY-selection signals Y0 and Y1 are both Low level, and the operating modeswitching signal RS is Low level.

Because the Y-selection signals Y0 and Y1 are both Low level, the readswitch control signal CRSE is High level. Accordingly, the read columnswitch 26 is off, and the sense amplifier 22 and the bit line BT and thebit line BB are electrically disconnected from each other.

Because the read switch control signal CRSE is High level, the writeswitch control signal CWSE is Low level. Accordingly, the write columnswitch 25 is off, and the write driver 21 and the bit line BT and thebit line BB are electrically disconnected from each other.

Because the operating mode switching signal RS is Low level, theinverted operating mode switching signal RSI is High level, and thedelayed inverted operating mode switching signal RSI_D is High level.Accordingly, the resume mode return precharge signal RSPC is High level,and the resume standby return precharge circuit 24 is off.

Because the inverted operating mode switching signal RSI is High leveland the delayed inverted operating mode switching signal RSI_D is Highlevel, the precharge signal PC is High level. Because the read switchcontrol signal CRSE is also High level, the precharge control signal CPCis Low level. Accordingly, the normal operation precharge circuit 23 ison, and the bit line BT and the bit line BB are precharged to Highlevel.

As described above, in the NOP state in the normal operation mode, thebit line BT and the bit line BB are kept at High level by the normaloperation precharge circuit 23. Note that, in this state, because thesources of the NMOS transistors N3 and N4 in the memory cell 1 aregrounded (ground potential VSS), a leakage current flows from the powersupply to the ground due to a channel leakage of the load (PMOStransistors P1 and P2), the drive transistor (NMOS transistors N3 andN4) and the transfer transistor (NMOS transistors N1 and N2). Further, aleakage current flows from the bit line (power supply) to the substrateof the transfer transistor (ground) due to GIDL of the transfertransistor.

Next, the operation in the case of transition from the normal operationmode to the resume standby mode (timing T1 in FIG. 4) is described.While the word line WL stays Low level, the operating mode switchingsignal RS transitions from Low level to High level. Note that, in theresume standby mode, the power supply of an address decoder (not shown)is cut, the Y-selection signals Y0 and Y1 are indeterminate.

The inverted operating mode switching signal RSI transitions from Highlevel to Low level. Although the Y-selection signals Y0 and Y1 areindeterminate, because the PMOS transistor P71 turns on, the read switchcontrol signal CRSE is driven to High level. Accordingly, the readcolumn switch 26 is off, and the sense amplifier 22 and the bit line BTand the bit line BB are electrically disconnected from each other.

Because the read switch control signal CRSE is High level, the writeswitch control signal CWSE is Low level. Accordingly, the write columnswitch 25 is off, and the write driver 21 and the bit line BB areelectrically disconnected from each other.

Even when the inverted operating mode switching signal RSI transitionsfrom High level to Low level, the resume mode return precharge signalRSPC remains High level, and the resume standby return precharge circuit24 is off.

When the inverted operating mode switching signal RSI transitions fromHigh level to Low level, the precharge signal PC becomes Low level.Accordingly, the precharge control signal CPC becomes High level, andthe normal operation precharge circuit 23 is off.

Because the word line WL is Low level, the transfer transistor is off.

As described above, in the resume standby mode, the bit line BT and thebit line BB are electrically disconnected from the other circuits in thesemiconductor storage device 100 and are in floating state. Therefore,the potential of the bit line BT and the bit line BB is determined to bethe potential at which a leakage current of the memory cell 1 and theother circuits is the smallest. It is therefore possible to reduce aleakage current from the bit line to the substrate of the transfertransistor due to GIDL.

Even when the inverted operating mode switching signal RSI transitionsfrom High level to Low level, the delayed inverted operating modeswitching signal RSI_D does not immediately transition from High levelto Low level. Specifically, the delayed inverted operating modeswitching signal RSI_D transitions from High level to Low level afterthe lapse of a certain delay time from when the inverted operating modeswitching signal RSI transitions from High level to Low level.

As described above, in the resume standby mode, the bit line BT and thebit line BB are in floating state, and therefore a leakage current fromthe bit line to the substrate of the transfer transistor due to GIDL canbe reduced.

Next, the operation in the case of returning from the resume standbymode to the normal operation mode (timing T2 in FIG. 4) is described.While the word line WL stays Low level, the operating mode switchingsignal RS transitions from High level to Low level. Note that, the powersupply of the address decoder is also cut, though not shown. Therefore,the Y-selection signals Y0 and Y1 are indeterminate. After a certainperiod of time at which the power supply is restored, the Y-selectionsignals Y0 and Y1 become Low level.

The inverted operating mode switching signal RSI transitions from Lowlevel to High level. Although the Y-selection signals Y0 and Y1 areindeterminate initially, they transition to Low level after a certainperiod of time, and the read switch control signal CRSE is driven toHigh level. Accordingly, the read column switch 26 is off, and the senseamplifier 22 and the bit line BT and the bit line BB are electricallydisconnected from each other.

Because the read switch control signal CRSE is High level, the writeswitch control signal CWSE is Low level. Accordingly, the write columnswitch 25 is off, and the write driver 21 and the bit line BT and thebit line BB are electrically disconnected from each other.

Even when the inverted operating mode switching signal RSI transitionsfrom Low level to High level, the delayed inverted operating modeswitching signal RSI_D does not immediately transition from Low level toHigh level.

When the inverted operating mode switching signal RSI becomes Highlevel, the resume mode return precharge signal RSPC becomes Low level.Accordingly, the resume standby return precharge circuit 24 turns on,and the bit line BT and the bit line BB are precharged to High level.

Because the delayed inverted operating mode switching signal RSI_D doesnot immediately transition from Low level to High level, even when theinverted operating mode switching signal RSI becomes High level, theprecharge signal PC is kept at Low level. Accordingly, the prechargecontrol signal CPC is kept at High, and the normal operation prechargecircuit 23 is also kept off. Therefore, during the period when the bitline BT and the bit line BB are precharged for return by the resumestandby return precharge circuit 24, precharge by the normal operationprecharge circuit 23 is not performed.

After the lapse of a certain period of time from when the invertedoperating mode switching signal RSI transitions from Low level to Highlevel (timing T3 in FIG. 4), the delayed inverted operating modeswitching signal RSI_D transitions from Low level to High level. As aresult, the resume mode return precharge signal RSPC transitions to Highlevel, and the resume standby return precharge circuit 24 turns off.

On the other hand, the precharge signal PC transitions to High level.Because the read switch control signal CRSE is High level, the prechargecontrol signal CPC transitions to Low level, and the normal operationprecharge circuit 23 turns on. The semiconductor storage device 100thereby returns to the normal operation mode.

As described above, at the time of returning from the resume standbymode to the normal operation mode, the bit line BT and the bit line BBare charged to High level by the resume standby return precharge circuit24 for a certain period of time after the return. Then, after the lapseof the certain period, the circuit to precharge the bit line BT and thebit line BB changes from the resume standby return precharge circuit 24to the normal operation precharge circuit 23, and the return to thenormal operation mode is thereby completed.

As described above, at the time of returning from the resume standbymode to the normal operation mode, it is necessary to precharge both ofthe bit line BT and the bit line BB in the semiconductor storage device100, and therefore a larger current is required for precharge comparedwith in the normal operation mode. Because a large number of bit linesare included in the semiconductor storage device 100, if they areprecharged using the normal operation precharge circuit 23, a peakcurrent required for precharge at the time of returning from the resumestandby mode to the normal operation mode is large.

On the other hand, in the semiconductor storage device 100, the drivingforce of the resume standby return precharge circuit 24 is designed tobe smaller than the driving force of the normal operation prechargecircuit 23. It is thereby possible to suppress a peak current whenperforming precharge at the time of returning from the resume standbymode to the normal operation mode.

Accordingly, in this configuration, it is possible to prevent theoccurrence of a reliability defect such as power supply potential dropor electromigration compared with the case of using the normal operationprecharge circuit for precharge at the time of returning from the resumestandby mode to the normal operation mode.

Further, in this configuration, the bit line is in floating state duringthe resume standby mode as described above. It is thereby possible toreduce a leakage current due to GIDL flowing from the bit line to thesubstrate of the transfer transistor.

Second Embodiment

A semiconductor storage device 200 according to a second embodiment isdescribed hereinafter. FIG. 5 is a circuit diagram schematically showinga configuration of the semiconductor storage device 200 according to thesecond embodiment. As shown in FIG. 5, the semiconductor storage device200 includes a memory cell 1, an I/O circuit 4, and an operating modecontrol circuit 5.

The I/O circuit 4 has a configuration in which the resume standby returnprecharge circuit 24 is eliminated from the I/O circuit 2 described inthe first embodiment and further the normal operation precharge circuit23 and the column I/O control circuit 27 are replaced by a normaloperation precharge circuit 43 and the column I/O control circuit 47,respectively. The other configuration of the I/O circuit 4 is the sameas that of the I/O circuit 2 and thus not redundantly described.

The normal operation precharge circuit 43 has a configuration in whichthe sources of the PMOS transistors P31 and P32 in the normal operationprecharge circuit 23 are connected to a precharge power supply line PSLinstead of VDD.

The column I/O control circuit 47 has a configuration in which the NANDcircuit 271 in the column I/O control circuit 27 is changed to aninverter 471. The input terminal of the inverter 471, the drain of thePMOS transistor P71, and the gates of the PMOS transistors P61 and P62in the read column switch 26, the output terminal of the NAND circuit272, and the input terminal of the inverter 273 are connected to eachother. The output terminal of the inverter 471 is connected to the gatesof the PMOS transistors P31 to P33 in the normal operation prechargecircuit 43 and outputs a precharge control signal CPC. The otherconfiguration of the column I/O control circuit 47 is the same as thatof the column I/O control circuit 27 and thus not redundantly described.

The operating mode control circuit 5 includes an inverter 31, a delaycircuit 32, an OR circuit 51, an NMOS transistor N5 and a PMOStransistor P5.

The inverter 31 and the delay circuit 32 are the same as those of theoperating mode control circuit 3 described in the first embodiment.

The power supply potential VDD is applied to the drain and the gate ofthe NMOS transistor N5. The source of the NMOS transistor N5 isconnected to the precharge power supply line PSL. The power supplypotential VDD is applied to the source of the PMOS transistor P5. Thedrain of the PMOS transistor P5 is connected to the precharge powersupply line PSL.

One input terminal of the OR circuit 51 is connected to the outputterminal of the inverter 31 and receives the inverted operating modeswitching signal RSI. The other input terminal of the OR circuit 51 isconnected to the output terminal of the delay circuit 32 and receivesthe delayed inverted operating mode switching signal RSI_D. The outputterminal of the OR circuit 51 is connected to the gate of the PMOStransistor P5.

The operation of the semiconductor storage device 200 is describedhereinafter. The timing of signals in the semiconductor storage device200 is the same as shown in FIG. 4.

In the NOP state in the normal operation mode, the PMOS transistor P5and the NMOS transistor N5 are on, and the power supply potential VDD isapplied to the precharge power supply line PSL.

On the other hand, in the resume standby mode (timing T1 in FIG. 4), thePMOS transistor P5 is off and the NMOS transistor N5 is on. Accordingly,a voltage that is lower than the power supply potential VDD by Vth(threshold) of the NMOS transistor N5 is applied to the precharge powersupply line PSL. Because the precharge control signal CPC is High level,the normal operation precharge circuit 43 is off, and the bit line BTand the bit line BB are in floating state.

In the case of returning from the resume standby mode to the normaloperation mode (timing T2 in FIG. 4), the PMOS transistor P5 is off andthe NMOS transistor N5 is on for a certain period of time. On the otherhand, because the precharge control signal CPC is Low level, the bitline BT and the bit line BB are precharged. At this time, the voltagethat is lower than the power supply potential VDD by Vth (threshold) ofthe NMOS transistor N5 is continuously applied to the precharge powersupply line PSL. Therefore, precharge of the bit lines is performedslowly, and it is thereby possible to reduce a peak current at the timeof precharge, just like the semiconductor storage device 100.

As described above, according to this configuration, when charging thebit lines at the time of returning from the resume standby mode to thenormal operation mode, the power supply potential applied to thecharging transistor in the normal operation precharge circuit 43 isdropped to reduce the driving capability of the charging transistor. Itis thereby possible to suppress a peak current at the time of return,just like the semiconductor storage device 100.

Therefore, in this configuration, the same advantageous effects as thesemiconductor storage device 100 according to the first embodiment canbe obtained.

Third Embodiment

A semiconductor storage device 300 according to a third embodiment isdescribed hereinafter. FIG. 6 is a block diagram schematically showing aconfiguration of the semiconductor storage device 300 according to thethird embodiment. As shown in FIG. 6, the semiconductor storage device300 has a configuration in which a word line driver 6 is added to thesemiconductor storage device 100.

FIG. 7 is a circuit diagram showing the word line driver 6 and thememory cell 1 according to the third embodiment. The word line driver 6includes a control signal generation circuit 61, a driver circuit 62, aresume standby word line holding circuit 63, a return word line powersupply switch 64, and a word line power supply switch 65.

The control signal generation circuit 61 includes inverters 611 to 613,a NOR circuit 614 and a NAND circuit 615. The inverted operating modeswitching signal RSI is input to the input terminal of the inverter 611from the operating mode control circuit 3. A return word line powersupply switch control signal LCM is output from the output terminal ofthe inverter 611. The inverted operating mode switching signal RSI isinput to one input terminal of the NOR circuit 614 from the operatingmode control circuit 3. The delayed inverted operating mode switchingsignal RSI_D is input to the other input terminal of the NOR circuit 614from the operating mode control circuit 3. The output terminal of theNOR circuit 614 is connected to the input terminal of the inverter 612and one input terminal of the NAND circuit 615. A word line power supplyswitch control signal LCMW is output from the output terminal of theinverter 612. The input terminal of the inverter 613 is connected to theoutput terminal of the inverter 612 and receives the word line powersupply switch control signal LCMW. An inverted word line power supplyswitch control signal LCMWI is output from the output terminal of theinverter 613 to the other input terminal of the NAND circuit 615. Aresume standby word line holding control signal LSMWD is output from theoutput terminal of the NAND circuit 615.

The return word line power supply switch 64 includes a PMOS transistorP6. The power supply potential VDD is applied to the source of the PMOStransistor P6. The drain of the PMOS transistor P6 is connected to aword line driver power supply line LCVDD. The return word line powersupply switch control signal LCM is input to the gate of the PMOStransistor P6.

The word line power supply switch 65 includes a PMOS transistor P7. Thepower supply potential VDD is applied to the source of the PMOStransistor P7. The drain of the PMOS transistor P7 is connected to theword line driver power supply line LCVDD. The gate of the PMOStransistor P7 is connected to the output terminal of the inverter 612and receives the word line power supply switch control signal LCMW.

The driver circuit 62 includes a PMOS transistor P11 and an NMOStransistor N11. The PMOS transistor P11 and the NMOS transistor N11 forman inverter circuit. The source of the PMOS transistor P11 is connectedto the drain of the PMOS transistor P6 in the return word line powerswitch 64 and the drain of the PMOS transistor P7 in the word line powersupply switch 65 (i.e. the word line driver power supply line LCVDD).The drain of the PMOS transistor P11 is connected to the drain of theNMOS transistor N11 and the word line WL. The source of the NMOStransistor N11 is grounded (ground potential VSS). A word line selectionsignal WLS is input to the gates of the PMOS transistor P11 and the NMOStransistor N11.

The resume standby word line holding circuit 63 includes an NMOStransistor N6. The drain of the NMOS transistor N6 is connected to theword line WL between the driver circuit 62 and the memory cell 1. Thesource of the NMOS transistor N6 is grounded (ground potential VSS). Thegate of the NMOS transistor N6 is connected to the output terminal ofthe NAND circuit 615 and receives the resume standby word line holdingcontrol signal LSMWD.

The operation of the semiconductor storage device 300 is describedhereinafter. FIG. 8 is a timing chart of signals in the semiconductorstorage device 300 according to the third embodiment. First, theoperation in the case of the NOP state in the normal operation mode isdescribed. In the NOP state in the normal operation mode, the operatingmode switching signal RS is Low level.

At this time, the delayed inverted operating mode switching signal RSI_Dis High level, and therefore the word line power supply switch controlsignal LCMW is Low level. Accordingly, the word line power supply switch65 is on and drives the word line driver power supply line LCVDD to Highlevel.

At this time, the inverted operating mode switching signal RSI is Highlevel. Accordingly, the return word line power supply switch controlsignal LCM is Low level, and the return word line power switch 64 is onand drives the word line driver power supply line LCVDD to High level.

Because the word line power supply switch control signal LCMW is Lowlevel, the inverted word line power supply switch control signal LCMWIis High level. Accordingly, the resume standby word line holding controlsignal LSMWD is Low level, and the resume standby word line holdingcircuit 63 is off.

As described above, in the normal operation mode, the word line driverpower supply line LCVDD is driven to High level by both of the word linepower supply switch 65 and the return word line power switch 64.

Next, the operation in the case of transition from the normal operationmode to the resume standby mode (timing T1 in FIG. 8) is described. Atthis time, the operating mode switching signal RS transitions from Lowlevel to High level.

Because the operating mode switching signal RS becomes High level, theword line power supply switch control signal LCMW becomes High level,and the word line power supply switch 65 turns off.

Because the inverted operating mode switching signal RSI becomes Lowlevel, the return word line power supply switch control signal LCMbecomes High level, and the return word line power switch 64 turns off.

Because the word line power supply switch control signal LCMW becomesHigh level, the inverted word line power supply switch control signalLCMWI becomes Low level. Accordingly, the resume standby word lineholding control signal LSMWD becomes High level, and the resume standbyword line holding circuit 63 turns on, and all of the word lines WL areheld at Low level.

As described above, in the resume standby mode, the word line driverpower supply line LCVDD is floating, thereby reducing a leakage currentin the driver circuit 62. Further, the word lines WL are held at Lowlevel by the resume standby word line holding circuit 63, instead of thedriver circuit 62.

Next, the operation in the case of returning from the resume standbymode to the normal operation mode (timing T2 in FIG. 8) is described. Atthis time, the operating mode switching signal RS transitions from Highlevel to Low level.

Even when the operating mode switching signal RS transitions to Lowlevel, the delayed inverted operating mode switching signal RSI_D doesnot immediately transition from Low level to High level. The word linepower supply switch control signal LCMW also does not immediatelytransition to Low level, and therefore the word line power supply switch65 remains off.

On the other hand, because the inverted operating mode switching signalRSI becomes High level, the return word line power supply switch controlsignal LCM immediately becomes Low level, and the return word line powerswitch 64 turns on, and the word line driver power supply line LCVDD ischarged to High level.

After a certain period of time from transition of the inverted operatingmode switching signal RSI from Low level to High level (timing T3 inFIG. 8), the delayed inverted operating mode switching signal RSI_Dtransitions from Low level to High level.

The word line power supply switch control signal LCMW thereby alsotransitions to Low level, and the word line power supply switch 65 turnson, and the word line driver power supply line LCVDD is driven to Highlevel.

As described above, at the time of returning from the resume standbymode to the normal operation mode, the word line driver power supplyline LCVDD is charged to High level by the return word line power switch64 for a certain period of time after the return. After that, the wordline power supply switch 65 turns on, and the return to the normaloperation mode is completed. The driving force of the return word linepower switch 64 is designed to be sufficiently smaller than the drivingforce of the word line power supply switch 65 in order to prevent anincrease in a peak current when charging the word line driver powersupply line LCVDD. Accordingly, the word line driver power supply lineLCVDD can be charged slowly compared with the case of using the wordline power supply switch 65 for charging. It is thereby possible toprevent the occurrence of an instantaneous voltage drop and areliability defect due to an increase in a peak current during charging.

Fourth Embodiment

A semiconductor storage device 400 according to a fourth embodiment isdescribed hereinafter. FIG. 9 is a circuit diagram schematically showinga configuration of the semiconductor storage device 400 according to thefourth embodiment. As shown in FIG. 9, the semiconductor storage device400 has a configuration in which a source level control circuit 7 isadded to the semiconductor storage device 100.

The source level control circuit 7 includes NMOS transistors N15 andN16. The drain and the gate of the NMOS transistor N15 are connected toa source line ARVSS. The drain of the NMOS transistor N16 is connectedto the source line ARVSS. The inverted operating mode switching signalRSI output from the operating mode control circuit 3 is input to thegate of the NMOS transistor N16. The sources of the NMOS transistors N15and N16 are grounded (ground potential VSS). The operation of thesemiconductor storage device 400 is described hereinafter. FIG. 10 is atiming chart of signals in the semiconductor storage device 400according to the fourth embodiment. The operation of the semiconductorstorage device 400 except the source level control circuit 7 is the sameas that of the semiconductor storage device 100 and thus not redundantlydescribed. The operation of the source level control circuit 7 isdescribed hereinbelow.

In the normal operation mode, the inverted operating mode switchingsignal RSI is High level. Thus, the source line ARVSS is driven to Lowlevel by the source level control circuit 7.

When transition occurs from the normal operation mode to the resumestandby mode (timing T1 in FIG. 10), the inverted operating modeswitching signal RSI transitions from High level to Low level. Becausethe inverted operating mode switching signal RSI is Low level, the NMOStransistor N16 in the source level control circuit 7 is off, and thesource line ARVSS is driven by the NMOS transistor N15 in diodeconnection. Accordingly, the potential of the source line ARVSS isdetermined by the ratio of a leakage current of the memory cell 1 and anon-current of the NMOS transistor N15 in diode connection. Therefore,the potential of the source line ARVSS rises to be higher than theground potential VSS, and it is thereby possible to reduce a leakagecurrent of the memory cell.

As described above, in this configuration, because the bit line BT andthe bit line BB are in floating state in the resume standby mode justlike in the semiconductor storage device 100, it is possible to reduce aleakage current from the bit line to the substrate of the transfertransistor due to GIDL.

Further, in this configuration, the potential of the source line ARVSSis raised to be higher than the ground potential VSS level by the sourcelevel control circuit 7 in the resume standby mode. It is therebypossible to reduce a leakage current due to a channel leakage as well.Therefore, further reduction of a leakage current can be achieved inthis configuration.

Other Embodiments

The present invention is not limited to the above-described embodiments,and various changes and modifications may be made without departing fromthe scope of the invention. For example, the I/O circuit 2 and theoperating mode control circuit 3 in the semiconductor storage deviceaccording to the third and fourth embodiments may be respectivelyreplaced by the I/O circuit 4 and the operating mode control circuit 5described in the second embodiment.

Further, both of the word line driver 6 and the source level controlcircuit 7 may be included in the semiconductor storage device accordingto the above-described embodiments.

The transistors described in the above embodiments are just examples.Various modifications, such as using other transistors or changingconductivity types, may be made as long as the same operation can beachieved.

Although embodiments of the present invention are described specificallyin the foregoing, the present invention is not restricted to theabove-described embodiments, and various changes and modifications maybe made without departing from the scope of the invention.

The above-described embodiments can be combined as desirable by one ofordinary skill in the art.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

What is claimed is:
 1. A semiconductor storage device comprising: anSRAM memory cell that is composed of a drive transistor, a transfertransistor and a load transistor; an I/O circuit that is connected tobit lines connected to the memory cell; and an operating mode controlcircuit that switches an operating mode of the I/O circuit between aresume standby mode and a normal operation mode, wherein the I/O circuitincludes: a write driver that writes data to bit lines, a senseamplifier that reads data from the bit lines, a first switch that isinserted between the bit lines and the write driver, a second switchthat is inserted between the bit lines and the sense amplifier, aprecharge circuit that precharges the bit lines, and a control circuitthat controls the first and second switches and the precharge circuitaccording to a signal from the operating mode control circuit, and thecontrol circuit turns off the first and second switches and theprecharge circuit in the resume standby mode, and causes the prechargecircuit to precharge the bit lines with a smaller driving force comparedwith in the normal operation mode when returning from the resume standbymode to the normal operation mode.
 2. The semiconductor storage deviceaccording to claim 1, wherein the precharge circuit includes: a firstprecharge circuit that precharges the bit lines in the normal operationmode, and a second precharge circuit that has a smaller driving forcethan the first precharge circuit and precharges the bit lines, and thecontrol circuit causes the second precharge circuit to precharge the bitlines when returning from the resume standby mode to the normaloperation mode.
 3. The semiconductor storage device according to claim2, wherein the control circuit turns off the second precharge circuitafter precharge of the bit lines is completed, and causes the firstprecharge circuit to precharge the bit lines in the subsequent normaloperation mode.
 4. The semiconductor storage device according to claim1, wherein the operating mode control circuit supplies a power supplypotential to the precharge circuit, and when returning from the resumestandby mode to the normal operation mode, the operating mode controlcircuit reduces the power supply potential supplied to the prechargecircuit compared with in the normal operation mode.
 5. The semiconductorstorage device according to claim 4, wherein the operating mode controlcircuit raises the power supply potential supplied to the prechargecircuit to a potential used in the normal operation mode after prechargeof the bit lines is completed.
 6. The semiconductor storage deviceaccording to claim 1, comprising: a source potential control circuitthat is connected to a source line connected to a source of the drivetransistor and supplies a potential to the source line, wherein thesource potential control circuit sets a potential of the source line ata ground potential in the normal operation mode, and sets a potential ofthe source line at a potential higher than the ground potential in theresume standby mode.
 7. The semiconductor storage device according toclaim 1, comprising: a word line driver that drives a word lineconnected to the memory cell, wherein the word line driver includes: adriver circuit that supplies a potential to the word line, a powersupply line of the driver circuit, a first power supply switch that isinserted between the power supply line and a power supply, a secondpower supply switch that is inserted between the power supply line and apower supply and has a smaller driving force than the first power supplyswitch, and a control signal generation circuit that supplies a controlsignal to the first and second power supply switches, and the controlsignal generation circuit closes the first power supply switch and opensthe second power supply switch in the normal operation mode, opens thefirst and second power supply switches in the resume standby mode, andopens the first power supply switch and closes the second power supplyswitch when returning from the resume standby mode to the normaloperation mode.
 8. The semiconductor storage device according to claim7, wherein the control signal generation circuit closes the first powersupply switch and opens the second power supply switch after the wordline reaches a power supply potential.